The invention relates to a reluctance electric machine comprising
a stator part with stator teeth of magnetically conductive material that are provided with coil windings;
and a rotor part arranged coaxially with respect to the stator part and located opposite the stator part so as to leave free an air gap therebetween,
the rotor part having a number discrete poles of magnetically conductive material that project in the direction towards the stator part.
Reluctance electric machines differ from other electric machines in that they have no electromagnetic or permanent-magnetic excitation part. Reluctance electric machines operate by using the effect of magnetic attraction of magnetically conducting parts under the influence of magnetic flux flowing through are same. Both the stator and the rotor have distinctive magnetically conductive poles. The coil windings have current flowing therethrough such that they attract an adjacent rotor pole each. The current in the stator pole has to be turned off again at the proper time after attraction of the respective rotor pole, in order to release the rotor pole in running direction. For, if the coil current would continue to flow (irrespective of the direction of flow), continued attraction by the stator pole, i.e. a retroactive force, would result that would prevent further rotation of the rotor.
It is thus in the nature of reluctance electric machines that these are exploited basically half of the time only. This is why reluctance electric machinesxe2x80x94assuming otherwise identical design parametersxe2x80x94normally achieve only about half of the torque values or power values in comparison with other electric motors.
It is the object of the invention to improve a reluctance electric machine of the type indicated at the outset towards higher values as regards permanent torque and permanent power.
To meet this object, the reluctance electric machine according to the invention is characterized in that cooling with channelled coolant flow is provided at least for partial sections of the coil windings of the stator part.
Preferably, the coil windings are provided with an enclosure each. As an alternative, there are preferably several coil windings commonly provided with an enclosure each. In accordance with a particularly preferred embodiment, the stator part is provided with an enclosure in its entirety. In all of the cases mentioned, there may be provided, for the space enclosed by the enclosure, one or more coolant supplies and one or more coolant discharges.
As regards the term xe2x80x9cprojecting, discrete polesxe2x80x9d in feature (c) of claim 1, it is to be pointed out furthermore that the portions between the discrete poles may also be filled with a non-magnetic material so that the rotor part is virtually smooth on the air gap side.
The invention teaches to make the cooling of the stator part considerably more efficient than with known reluctance electric machines. The electric machine thus can be subjected to considerably higher loadsxe2x80x94permanently and not only for short periods, as before.
The statements further below will make clear that the channelling of the coolant flow according to the invention, in practical application, mostly has the effect that the stator part of the reluctance electric machine is provided with a sealing layer on the side facing the air gap. The result of the sealing layer is that the distance between the stator poles and the rotor poles across the air gap is greater than without provision of a sealing layer. This meansxe2x80x94with otherwise unchanged design parametersxe2x80x94a reduction of the magnetic flux between the respective pair of stator pole and rotor pole in consideration and thus a deterioration of the magnetic conditions of the electric machine. With the corresponding designs of the reluctance electric machine according to the invention, which have a sealing layer, this disadvantage is deliberately tolerated; according to the invention, the considerably more efficient cooling achieves a greater advantage than the deterioration of the magnetic conditions due to additional material in the air gap.
These restrictions are not applicable when the stator poles, by way of suitable enclosures, have individually associated cooling spaces or cooling channels that cover only the end face portions and do not project into the air gap.
Known reluctance electric machines also have already made use of cooling means. Known concretely are the non-specific blowing of air through the entire electric machine (stator part, air gap, rotor part) or as cooling on the stator part side directed away from the air gap. In contrast thereto, the reluctance electric machine according to the invention employs direct or quasi-direct cooling of those portions where the heat losses arise in the first place. A first major portion of generation of heat losses are the coil windings (often referred to as xe2x80x9ccopper lossesxe2x80x9d; these are caused mainly by the passage of current). According to the invention, the coil windings are cooled directly by cooling medium flowing therealong. Even if the coil windings are cooled only in the spaces between the stator teeth or only in the spaces on the face side of the stator teeth (and not in both spaces), very efficient cooling is achieved since the material of the coil windings, which has good current conducting properties, also provides for good conduction of the heat losses to the portions cooled concretely. Another major place of generation of heat losses are the stator teeth (often referred to as xe2x80x9ciron lossesxe2x80x9d in simplified manner; these losses are caused mainly by the continuous alternation between magnetization and demagnetization or remagnetization of the stator teeth). The described cooling operation by flow of cooling medium through spaces accommodating coil windings of the stator part, at the same time constitutes cooling of the stator teeth sincexe2x80x94at least with many design types of the coil windingsxe2x80x94the cooling medium flows in the spaces mentioned so as to reach the stator teeth and since the coil windings establishing physical contact with the stator teeth dissipate heat from the stator teeth by thermal conduction. Furthermore, the cooling medium usually flows along the stator back between the stator teeth; the stator back receives heat from the stator teeth by thermal conduction.
A preferred development of the invention provides that the stator teeth have internal flow passages for cooling medium. This provides for direct, particularly efficient internal cooling of the stator teeth. Furthermore, it is pointed out as preferred possibility according to the invention to provide in addition to, or instead of, the described internal cooling of the stator teeth, internal flow passages for cooling medium in the region of the stator part which are set back from the stator teeth.
It is expressly pointed out that the term xe2x80x9creluctance electric machinexe2x80x9d used in the present application comprises both electric motors (conversion of electric energy to mechanical energy) as well as current generators (conversion of mechanical energy to electric energy). In addition thereto, it is expressly pointed out that the term xe2x80x9cstator partxe2x80x9d is not supposed to mean that the stator part cogently is to be non-moving, and that the term xe2x80x9crotor partxe2x80x9d is not supposed to mean that the rotor part cogently is supposed to be a rotatable part of the electric machine. Rather, it is easily possible to provide the stator part (i.e. the part of the electric machine provided with coil windings) as rotating machine part and the rotor part as stationary, non-moving machine part. The inverse design, however, is usually more favorable since the current windings having current supplied thereto and, in case of the generator, discharged therefrom, are located on a stationary machine part, whereby sliding contacts are avoided. In addition thereto, the possible embodiment is to be mentioned that both the stator part and the rotor part rotate at different speeds and/or different directions of rotation, in particular when they are connected to each other via a gear system or part of a gear system, e.g. a planetary gear system.
The electric machine according to the invention preferably is formed with electronic activation and deactivation of the coil winding, currents. By means of a suitable sensor or on the basis of electric information from feeding the electric motor, the rotational relative position between stupor part and rotor part is ascertained; the current through the coil windings is turned on and off again at the proper time each. Suitable electric circuits and suitable electronic components for realizing the described current control for electric motors and current withdrawal for generators, respectively, are known and need not be described here in more detail.
The reluctance electric machine according to the invention may be designed to have axe2x80x94roughly speakingxe2x80x94cylindrical air gap (so that cooperating pairs of stator pole and rotor pole, so to speak, are xe2x80x9cconfrontingxe2x80x9d each other in radial direction) orxe2x80x94roughly speakingxe2x80x94an air gap located in a plane perpendicularly to the axis of rotation of the rotor part (so that cooperating pairs of stator pole and rotor pole, so to speak, are xe2x80x9cconfrontingxe2x80x9d each other in axial direction). It is possible in both cases to make use of several air gap portions, i.e. for example several cylindrical air gap portions on different diameters of the electric machine or several xe2x80x9cplanarxe2x80x9d air gap portions axially beside each other; in both cases there is formed a sequence of stator partxe2x80x94rotor partxe2x80x94stator partxe2x80x94rotor partxe2x80x94stator part etc.
In a preferred embodiment of the invention, the stator part is provided with a sealing layer on the side facing the air gap. By means of the sealing layer, it is possible in particularly simple manner in terms of construction to seal the spaces accommodating the coil windings in the direction towards the air gap in tight manner with respect to the cooling medium. For sealing the spaces on the face side of the stator teeth in the other directions so as to provide tightness with respect to the cooling medium, it is possible in particular to provide for each stator tooth a component part of U-shaped cross-section. These circumstances will be described in more detail further below in the description of a concrete embodiment.
In case of thexe2x80x94roughly speakingxe2x80x94cylindrical air gap, the sealing layer normally is cylindrical or substantially cylindrical. In case of the xe2x80x9cplanarxe2x80x9d air gap arranged perpendicularly to the axis of rotation, the sealing layer normally is planar or substantially planar and, in a front view, has substantially the shape of a ring of a circle.
Preferably, the sealing layer has a first layer for fulfilling the sealing function and a second layer for taking up the forces acting on the sealing layer. The first layer may preferably by a plastics film. The second layer may preferably consist of plastics material, in particular fiber-reinforced plastics material (e.g. reinforced with carbon fibers). As regards the forces acting on the sealing layer, especially the forces due to excess pressure of the cooling medium and centrifugal forces due to rotation of the stator part (with a machine design with rotating stator part) should be mentioned. The thickness of the sealing layer is preferably 0.3 to 1.5 mm.
The reluctance electric machine according to the invention may use either liquid cooling medium or gaseous cooling medium. Liquid cooling medium as a rule leads to higher cooling efficiency.
In case of the design withxe2x80x94roughly speakingxe2x80x94cylindrical air gap, it is possible to select either a design with external rotor (the rotor part has its rotor poles located radially farther outside than the air gap) or a design with internal rotor (the stator part has its stator teeth located radially farther outside than the air gap). In the former case, the rotor part often is of pot-shaped configuration in its entirety. The same holds for rotors having the afore-mentioned multiple air gap of cylindrical configuration.
Preferably, the stator part, on the side directed away from the rotor part, is designed such that the heat transfer is increased. This can be achieved by increasing the surface area or also and in addition by the generation of turbulent flow. Heat dissipation ribs are indicated as a concrete example in this respect. On the xe2x80x9crear sidexe2x80x9d of the stator part, cooling can be effected either with the same cooling medium ad the one flowing through the flow passages associated with the coil windings. As an alternative, it is also possible to use a different cooling medium on this xe2x80x9crear sidexe2x80x9d, e.g. cooling air supplied by a fan or air without forced movement.
Preferably, the coil windings in the winding head portions located on the face side of the stator teeth, are provided with flow passages for the cooling medium that are left free between oil winding conductors. While the coil winding in the grooves between the stator teeth normally should be wound as densely packed as possible for electric and magnetic reasons, in particular for reducing losses, it is more tolerable in the winding head portions located on the face side of the stator teeth to arrange either individual coil winding conductors in spaced apart manner or, as a further example, to leave small distances between winding layers, e.g. in axial direction. The measures mentioned serve to intensify the heat transfer in the winding head portions mentioned.
In a preferred development of the invention, the coil windings of the stator part are in the form of individual coils that are not interlinked with respect to the magnetic flux. This is conceivable, for example, such that a prefabricated individual coil is slid onto each individual stator tooth. The individual coils either are electrically connected to each other directly (e.g. in series or in parallel) or are connected individually or in groups to the electronic current control.
Preferably, there are provided a first, internal cooling circuit for circulating the cooling medium through the flow passages described, and a second, external cooling circuit for circulating another cooling medium, with the external cooling circuit being connected to the internal cooling circuit via a heat exchanger. The cooling medium in the internal cooling circuit and the additional cooling medium in the external cooling circuit may be the same cooling media. However, particularly preferred is a design in which the cooling medium in the internal cooling circuit is electrically non-conductive oil and the additional cooling-medium in the external cooling circuit is a different cooling liquid, in particular water. As a rule, there is provided an additional heat exchanger via which the external cooling circuit dissipates its heat to the environment.
Preferably, the internal cooling circuit has a circulation pump of its own. The internal cooling circuit and the heat exchanger preferably are integrated in terms of space on the reluctance electric machine. The external cooling circuit may, but does not have to, extend a distance away from the reluctance electric machine.
The described design with two coupled cooling circuits allows the necessary volume of cooling medium in the internal cooling circuit to be kept low. This is favorable since, as a general rule, special cooling medium has to be employed here which may establish contact with the portions of the electric machine subjected to the coolant, without disturbance being caused thereby. These specific cooling media, e.g. electrically non-conductive cooling oil, are comparatively expensive.
A concrete embodiment for an electric motor cooling means with internal and external cooling circuits is described in German patent application 196 51 119.4. The total contents of this laid open publication is made part of the instant application by making reference thereto. Preferably, the reluctance electric machine according to the invention has one or more features that are disclosed in said German patent application 196 51 119.4.